This invention relates to a polycrystalline film of a semiconductive material that is supersaturated with a deep level impurity. More specifically, this invention relates to a single phase polycrystalline film of semiconductive material that can be used as a temperature independent electrical resistor.
It is known that a resistor made of a semiconductive material having a shallow lever dopant will have a positive temperature coefficient of resistance over a narrow range of temperature near room temperature. This is due at least in part to a decrease in carrier mobility with increasing temperature within the depletion range of the doped semiconductive material. Accordingly, if uncompensated, a semiconductive resistor will increase in resistance value in this range with increase in temperature.
Considerable effort has been devoted to tailoring the temperature coefficient of resistance for semiconductor resistors. Much of it has been directed to compensating the resistance change, to maintain it substantially stable over a selected temperature range. In general, the tailored semiconductive resistors involve a monocrystalline semiconductive host material containing a shallow level dopant and a deep level dopant. The two dopants are present in selected combinations and relative concentrations to obtain the selected temperature characteristic within a given temperature range. Semiconductor resistors have been produced in which the inherent increase in resistivity is partially and even wholly compensated, or in some instances augmented. In some instances, as for example as disclosed in U.S. Pat. No. 3,292,129 Sanchez et al, a semiconductor host material can be more than compensated. It can be converted into having a negative temperature coefficient of resistance, forming an NTC thermistor. U.S. Pat. No. 3,248,677 Hunter et al discloses including both shallow level and deep lever dopants in a semiconductor host material to obtain more complete temperature compensation over a limited temperature range.
U.S. Pat. No. 3,484,668 Komatsu reports that full compensation cannot actually be obtained. The solid solubility of a deep level impurity in the semiconductive host material limits one from obtaining maximum compensation of mobility decrease with temperature. Consequently another approach is described by Komatsu to get adequate compensation in the selected temperature range. Prior semiconductor resistors require precise moderate concentrations for both the dominant shallow level dopant and the compensating deep level dopant. Moderate levels of doping are difficult to reproduce with accurate precision, particularly on polycrystalline material. This problem is aggravated considerably when two precise dopings are required. Accordingly, yields are lower and costs increased. Costs are also increased if the resistor is made with single crystal material. Lastly, most semiconductor resistors exhibit their selected tailored properties over only a narrow temperature range that only includes some but not all of the range -40.degree. C. to 100.degree. C.